The ability of Omicron to escape the immune response to previous infections and vaccinations has demonstrated the ability of the coronavirus to adapt in ways that allow it to maintain its proliferation in the population. An analysis on re-infections recorded in Qatar was recently published as a letter to the publisher in the New England Journal of Medicine and assesses whether a previous infection protects against possible re-infections.
Professors of the Medical School of the National and Kapodistrian University of Athens, Gikas Majorkinis and Thanos Dimopoulos (Rector of EKPA) report that the study showed, like other similar studies, the highest ability of Omicron to escape re-infection. More specifically for the strains’ Alpha, Beta and Delta the previous infection protected from symptomatic re-infection at levels higher than 85%, while with the mutation Ομικρον this level drops to 56%. However in the same analysis it seems that significant protection against serious disease and Covid-19 is offered although the number of seriously ill in this study was very small.
The findings of studies on the rate of re-infection are important in calculating the likelihood of achieving immunity a herd in a natural way.
Herd immunity is the phenomenon where the transmission of a pathogen to the population is prevented by those who have immunity (either natural or by vaccination) to such an extent that the epidemic stops. The theory predicts that this happens when the number of transmissions of each vector in the general population, ie the active number of reproduction, also known as Rt, falls below the unit.
For this to happen, a percentage of the population must have sterile immunity, that is, they must not catch the virus or, if it does, it must not transmit it.
This percentage is roughly calculated by the basic number of reproduction, known as R0, ie the number of transmissions that each vector makes in a population that has no immunity at all.
The baseline reproduction number in the original strains of SARS-CoV-2 was 2.5-3.0 and corresponds to a need for a sterilization immunity rate of 60 to 67%. The estimate for the Alpha, Beta and Delta strains is that the basal number of breeds has increased to about 5 for the Delta variant so that the sterilization immunity required for herd immunity reaches 80%.
As it turns out, such a rate of sterile immunity based on data published by the study (which shows that Delta variant infection reaches 90%) could only be achieved when the disease in the population reached 88% of the population.
So the reason we started to see a slowdown in the epidemic in Greece in early December was that the combination of natural disease and vaccination was approaching 88% leading to de-escalation of the epidemic, say the two Professors.
With the advent of Omicron, however, even if we accept that there is no increase in baseline reproduction, the maximum sterilization immunity that can be achieved is 56%, so there could be no herd immunity even if 100% of the population was infected with previous executives.
It should be noted, however, that these re-infection rates refer to infection with different strains and not strains of the same variant (eg first Delta and then Omicron).
Therefore we do not know the rate of Omicron to Omicron re-infection which will logically be higher than 56%. In any case, re-infection data show that the targets of SARS-CoV-2 vaccination programs have been revised upwards by rates that exceed 90% for all adults and reach 100% for those over 60 years of age.
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